1. Field of the Invention
The present invention relates to an objective lens driving apparatus, and in particular to an objective lens driving apparatus for use in an optical disc drive for recording on an optical disc or reproducing data from the optical disc while the optical disc is rotating.
2. Description of the Related Art
The above-described type of optical disc drives are provided with an objective lens driving apparatus for moving an objective lens in a focusing direction and a tracking direction. In this specification, the focusing direction refers to the direction perpendicular to a surface of the optical disc, and the tracking direction refers to a direction horizontal and parallel to the radial direction of the optical disc.
Briefly referring to FIGS. 13 and 14, a conventional objective lens driving apparatus 100 will be described. FIG. 13 is a perspective view of the objective lens driving apparatus 100, and FIG. 14 is a side view of the objective lens driving apparatus 100. In this specification, the focusing direction and the tracking direction are set as shown in FIG. 13.
As is illustrated in FIGS. 13 and 14, an objective lens 11 is supported by an objective lens holder 12 in a state where the optical axis thereof is perpendicular to the surface of an optical disc (not shown). In such a state, the objective lens 11 converges an optical beam on the optical disc. The objective lens holder 12 is attached to a carriage 31 via four springs 13. The four springs 13 support the objective lens holder 12 in such a manner that the objective lens holder 12 is movable in the focusing direction and the tracking direction. To the objective lens holder 12, a focusing coil 14 and a tracking coil 15 are attached as well as the objective lens 11. The focusing coil 14, the tracking coil 15, and a pair of magnets 23 are included in an actuator for moving the objective lens holder 12 in the focusing direction and the tracking direction. The magnets 23 are attached to a yoke 21 which is fixed to the carriage 31. The magnets 23 and the yoke 21 are included in a magnetic circuit 26.
The objective lens driving apparatus 100 having the above-described structure operates in the following manner.
While the data is recorded on the optical disc or the data is reproduced from the optical disc, appropriate focusing servo control and tracking servo control are performed on the actuator. The carriage 31 moves in the tracking direction by a guiding mechanism and a driving mechanism (not shown), for example, a chassis or a spindle motor.
When a driving current flows through the focusing coil 14, driving forces F1 and F2 (FIG. 14) are generated and act on the focusing coil 14 and the magnetic circuit 26, respectively. The driving forces F1 and F2 have the same magnitude and act in opposite directions from each other. The driving force F1 moves the objective lens holder 12 in the focusing direction and is conveyed to the carriage 31 via the springs 13, thereby, vibrating the carriage 31. The driving force F2 vibrates the yoke 21 of the magnetic circuit 26 and the carriage 31 on which the yoke 21 is fixed.
The vibration of the carriage 31 caused by the driving forces F1 and F2 is conveyed to the optical disc through the guiding mechanism and the driving mechanism. If such a path for conveying the vibration has a specific resonance point, focusing servo control is impossible. Such a problem, which occurs more easily and more frequently today when less rigid components are used in the optical disc drives in order to reduce the size and weight of the drives, is a barrier against further reduction in the size and weight of the drives.
Japanese Laid-Open Patent Publication No. 4-32035 proposes, as one of the solutions of such a problem, insertion of a visco-elastic body such as rubber or a spring into the conveying path of the vibration in order to damp the vibration. Such a method has problems in that, for example, positioning precision of the components of the objective lens driving apparatus is lowered. Especially the method, described in the above-mentioned publication, of visco-elastically supporting the magnetic circuit by rubber or a cantilever spring tends to present problems in that sufficient damping is not obtained, and in that the magnetic circuit is inclined by the movement of the objective lens holder and thus contacts the coil.
Japanese Laid-Open Patent Publication No. 4-245033 proposes provision of a device for generating a vibration having the same magnitude with and the opposite phase to the vibration generated by the movement of the objective lens holder in order to counteract the vibration. Provision of such a device increases the number of the components and the size of the apparatus.